Sewage treatment systems in common use employ the well known biological process utilizing aerobic or anaerobic digestion. These systems rely on bacterial action to break down organic materials and in so doing the solids are reduced to inactive sludge, and the liquid is rendered aseptic and suitable for disposal.
In a typical sewage treatment plant all sewage wastes, solids and liquid, are transmitted by conduits to a centrally located treatment plant. Solid materials are broken up into fine particles and transported to collecting and settling tanks where the solids, in the form of a sludge, are allowed to settle to the bottom. Bacterial action in the activated sludge is frequently improved by agitation, injection of oxygen, heating, and other means. The time required to treat sewage is measured in days. A great deal of equipment such as settling basins, clarifiers, filters, chlorinators, etc., is required to produce final effluent suitable for disposal in rivers, streams, or lakes.
There are many undesirable aspects of presently available biological sewage treatment systems. A large network of sewage pipes must be utilized to convey sewage to the treatment plant. Such piping systems employ large diameter conduit, and where gravity flow is not possible pumping stations must be provided.
The conventional sewage treatment plants require a great deal of space for settling basins, clarification tanks and ancillary equipment. When storm drains are connected with the sewer mains an additional liquid load is imposed on the plant during rainfall.
As cities expand outward away from the centrally located treatment plant the cost of additional sewer lines goes up, and frequently existing sewers can not handle the increased load. In large municipalities sewage must be piped miles to the treatment plant.
Biological treatment is time consuming, and the long time required for treatment necessitates a large retention capacity.
Biological treatment systems as presently known do not lend themselves readily to small isolated installation in areas of low population density because of cost, space requirements, and maintenance.